Filtration control is one of the most important properties of a drilling fluid, particularly when drilling through permeable formations where the hydrostatic pressure exceeds the formation pressure. It is important for a drilling fluid to quickly form a filter cake which effectively minimizes fluid loss, but which also is thin and dispersible enough to allow product to flow into the wellbore during production.
Filtration control additives for brines typically are nonionic water soluble polymers, such as starches, derivatized starches, gums, derivatized gums, and cellulosics. These polymers have certain advantages, but suffer from the disadvantage that they have a relatively low hydration rate in brines—particularly in high density brines, where very little water actually is available to hydrate and swell the polymers.
Another disadvantage of nonionic water-soluble polymers is that they have limited temperature stability. As wells are drilled deeper, higher bottomhole temperatures are encountered. Today's drilling fluids need to maintain stable rheology and low filtration at temperatures above 300° F. Unfortunately, the nonionic water soluble polymers currently in use are not stable at temperatures exceeding about 225° F. with extended aging times.
Filtration control additives are needed which will quickly form a thin, dispersible filter cake, and which also have high temperature stability for prolonged periods of time.